1. Field of the Invention:
The present invention relates to novel ceric oxide particulates having improved morphological characteristics, including a large specific surface at an elevated temperature, and to a process for the production of such novel ceric oxide particulates.
In the description that follows, by the term "specific surface" is intended the BET specific surface determined by the adsorption of nitrogen according to ASTM Standard D 3663-78, established by the BRUNAUER-EMMETT-TELLER method described in the Journal of the American Chemical Society, 60, 309 (1938).
2. Description of the Prior Art:
It is known to this art that ceric oxide is a useful catalyst or catalyst support. Compare, for example, the work of Paul Meriaudeau et al relative to the synthesis of methanol from CO+H.sub.2 on catalysts comprising platinum deposited onto ceric oxide (C.R. Acad. Sc. Paris, Vol. 297--Series II-471 (1983)).
It too is known to this art that the effectiveness of a catalyst is typically a function of the amount of the surface area of the catalyst in contact with the reagents. Thus, it is necessary to provide and maintain the catalyst in the highest possible divided state, i.e., the solid particles comprising same should be as small and individualized as possible. The fundamental role of the support is thus to maintain the catalyst particles or crystallites in contact with the reagents, in the highest possible divided state.
Upon extended use of the catalyst support, its specific surface is reduced due to the coalescence of the very fine micropores. Over the course of such coalescence, a portion of the volume of the catalyst is merged into the mass of the support and is no longer able to be contacted by the reagents.
Heretofore, most of the known ceric oxides had a specific surface which decreased rapidly at operating temperatures higher than 500.degree. C. Thus, R. Alvero et al (J. Chem. Soc., Dalton Trans., 87 (1984)) prepared a ceric oxide from ammonium cerinitrate having, after calcination at 600.degree. C., a specific surface of 29 m.sup.2 /g.
FR-A 2,559,754 further describes a ceric oxide having a specific surface of at least 85.+-.5 m.sup.2 /g prepared by calcination at a temperature ranging from 350.degree. to 450.degree. C., preferably having a specific surface of from 100 to 130 m.sup.2 /g after calcination at a temperature of from 400.degree. to 450.degree. C. This oxide is prepared by the hydrolysis of an aqueous solution of ceric nitrate in a nitric acid medium, followed by separation of the precipitate thus formed, washing with an organic solvent, optional drying, and, finally, calcination. The final product ceric oxide has a desirable specific surface, when calcined at a temperature ranging from 300.degree. to 600.degree. C. However, a decrease in the specific surface is observed after calcination at a higher temperature, such specific surface being about 10 m.sup.2 /g after calcination at 800.degree. C.
In FR-A 2,559,755, a ceric oxide is described having a specific surface of at least 85.+-.5 m.sup.2 /g after calcination at a temperature of from 350.degree. to 500.degree. C., and preferably a specific surface ranging from 150 to 180 m.sup.2 /g after calcination at a temperature of from 400.degree. to 450.degree. C. This oxide is prepared by precipitating a basic ceric sulfate by reacting an aqueous solution of ceric nitrate with an aqueous solution containing sulfate ions, separating the precipitate thus formed, washing the precipitate with an ammonia solution, optionally drying it, and then calcining such precipitate at a temperature ranging from 300.degree. to 500.degree. C. The ceric oxide prepared in this manner has a large specific surface, but when subjected to a calcining operation at 800.degree. C., its specific surface decreases considerably, to about 10 m.sup.2 /g.
In EP 88/401,593.4 (corresponding to European Patent No. 300851 B granted Feb. 26, 1992 and U.S. Pat. Nos. 5,011,671; 5,023,070 and 5,174,984), assigned to the assignee hereof, a process is described for increasing and stabilizing the specific surface of a ceric oxide at an elevated temperature.
Such process entails exposing the ceric oxide or precursor of ceric oxide to a solvothermal treatment prior to the calcining operation.
More particularly, such process comprises:
(a) suspending ceric hydroxide in a liquid medium; PA1 (b) heating the suspension in an enclosure to a temperature and pressure respectively lower than the critical temperature and critical pressure of said medium; PA1 (c) cooling the reaction medium and permitting it to return to atmospheric pressure; PA1 (d) separating the ceric hydroxide treated in this manner; and PA1 (e) calcining the separated ceric hydroxide. PA1 (a) suspending, in an aqueous solution of a decomposable base, a ceric hydroxide of the general formula (I): EQU Ce(M).sub.x (OH).sub.y (NO.sub.3).sub.z (I) PA1 (b) heating the resulting suspension in a closed enclosure, or confined reaction zone, to a temperature and pressure respectively lower than its critical temperature and pressure (the autoclaving operation); PA1 (c) cooling such reaction medium and permitting it to return to atmospheric pressure; PA1 (d) separating the ceric hydroxide treated in this manner; and PA1 (e) calcining said separated ceric hydroxide. PA1 (i) with r=4, cerium (IV) precipitates in a gelatinous form; PA1 (ii) with r=0, the cerium (IV) is in the ionic state; PA1 (iii) with 0&lt;r&lt;4, the cerium (IV) is in the ionic and/or colloidal state.
By "ceric hydroxide" is intended a hydrated ceric oxide CeO.sub.2.2H.sub.2 O, or a ceric hydroxide which may contain residual anions whether bonded or adsorbed, such as, for example, chlorides, sulfates, acetates, formates, and the like.
A preferred embodiment of EP 88/401,593.4 includes using a base solution as the liquid autoclaving medium.
Such a process makes it possible not only to increase the specific surface of the final product ceric oxide, but also same retains a large specific surface at temperatures of up to about 900.degree. C.
According to EP 88/401,594.2 (corresponding to European Patent No. 300852 B granted Sep. 18, 1991 and U.S. Pat. No. 5,174,984), by subjecting a ceric hydroxide prepared by reacting a cerium salt solution with a base, optionally in the presence of an oxidizing agent, at a pH greater than 7, to an autoclave treatment in a basic medium, a ceric oxide is produced having a specific surface at 800.degree. to 900.degree. C. never before attained in this art.
The ceric oxide thus produced has a specific surface of at least 15 m.sup.2 /g measured after calcination at a temperature of 800.degree. to 900.degree. C., such specific surface preferably ranging from 20 to 60 m.sup.2 /g measured after calcination at 800.degree. C.
It has a specific surface of 160 to 15 m.sup.2 /g measured after calcination at a temperature ranging from 350.degree. to 900.degree. C.
Therefore, it may have a specific surface of from 70 to 160 m.sup.2 /g, and preferably ranging from 100 to 160 m.sup.2 /g, measured after calcination at a temperature ranging from 350.degree. to 450.degree. C.
In the present application, the specific surfaces reported are measured on a final product calcined for at least 2 hours at the given temperature.
It has now unexpectedly and surprisingly been determined that in carrying out the process described in EP 88/401,593.4, but judiciously selecting the starting material ceric hydroxide and the liquid dispersion medium, a ceric oxide is prepared having still further improved morphological characteristics.